This invention relates to a polypropylene composition. More particularly it relates to a polypropylene composition affording, when molded, molded products having superior stiffness, transparency, impact resistance, resistance to whitening on impact, resistance to whitening on bending, luster, etc. 2. Description of the Related Art
Propylene homopolymer resins are relatively cheap and provided with physical properties satisfying performances required in the market; hence they have been broadly used as general-purpose resins in various molding fields such as film molding field, injection molding field, extrusion molding field, blow molding field, extrusion-stretching molding field, etc.
However, molded products obtained from the propylene homopolymer resins have a drawback that they are inferior in the impact resistance. In order to overcome this drawback, various compositions have so far been developed such as compositions obtained by blending an elastomer such as ethylene-propylene copolymer rubber, styrene-butadiene copolymer, etc. or propylene-ethylene block copolymer with propylene homopolymer, or compositions obtained by using propylene-ethylene block copolymer in place of propylene homopolymer. Further, in order to improve the transparency and impact resistance of propylene homopolymer, there has been developed a propylene polymer resin composition obtained by blending with a crystalline propylene polymer resin selected from among propylene homopolymer, propylene-.alpha.-olefin random copolymers and mixtures of the foregoing, a styrene-ethylene-butylene block copolymer in a quantity of 5 to 60% by weight based on the total weight of both the components (Japanese patent application laid-open No. Sho 58-210950/1983).
However, as to the polypropylene composition obtained by blending an ethylene-propylene copolymer rubber or a propylene-ethylene block copolymer with propylene homopolymer, when a molded product is obtained from the composition, the molded product is improved in the impact resistance, but drawbacks are instead raised that the molded product becomes white-turbid, its transparency is damaged and its resistance to whitening on impact and stiffness are notably lowered.
Further, compositions obtained by replacing propylene homopolymer by propylene-.alpha.-olefin block copolymers are effective for improving the impact resistance of molded products obtained from the compositions, but drawbacks are instead raised that the resulting molded products become white-turbid, their transparency is damaged and the stiffness and luster of the surface of the molded products are notably lowered. Further, in the fields of large size molded products whose molding materials have been being converted into plastics in recent years, such as fields of large size interior parts for automobiles, e.g. rear quarter panel, industrial large size panel, pallet, large size battery case, etc., the molded products have a large self-weight and a long fixing span so that problems of deformation of the molded products such as the surging phenomenon, warpage, shell blister, etc. of the products, and further, problems of insufficient firm feeling of the molded products and injury and abrasion of the surface of the molded products due to insufficient stiffness thereof also have caused a narrow bottleneck of commercialization of the molded products. Thus it is practically difficult to use a composition obtained from a propylene-.alpha.-olefin block copolymer which exhibits only a flexural modulus of about 10,000 Kg f/cm.sup.2, for producing large size molded products as described above, and also the thus obtained molded products are inferior in the transparency and surface luster.
In order to improve the stiffness, compositions obtained by blending an inorganic filler such as talc with propylene homopolymer or propylene-.alpha.-olefin copolymers have been developed, but while the stiffness is improved, the impact resistance and surface luster are reduced and the resistance to whitening on impact and the resistance to whitening on bending are also not improved; hence it is utterly impossible to use them for producing the above-mentioned large size molded products.
Further, the composition disclosed in the above-mentioned Japanese patent application laid-open No. Sho 58-210950/1983 is effective for improving the impact resistance and transparency of molded products obtained from the composition, but the stiffness thereof is notably reduced; hence it is difficult to use the composition for producing large size molded products as described above.
The present inventors have made extensive research for developing a polypropylene composition capable of being also used for producing large size molded products. As a result, we have found that a composition obtained by blending a styrene-ethylene butylene-styrene block copolymer (a copolymer consisting of polystyrene block, ethylene-butylene random copolymer block and polystyrene block) or a styrene-ethylene propylene block copolymer (copolymer consisting of polystyrene block and ethylene-propylene random copolymer block), each in a definite quantity with a specified propylene homopolymer or a composition obtained by further adding an ethylene-propylene copolymer rubber and/or a polyethylene each in a definite quantity with the abovementioned composition is superior in the stiffness (12,000 Kg f/cm.sup.2 or more in terms of flexural modulus), impact resistance (6 Kg f-cm/cm or more in terms of Izod impact strength) and transparency (50% or less in terms of haze value), and can afford molded products having a superior resistance to whitening on impact, resistance to whitening on bending and surface luster, and have completed the present invention based on this finding.